Method for transmitting/receiving a comp reference signal

ABSTRACT

Disclosed is a method for transmitting a CoMP reference signal for accurate channel estimation. The CoMP reference signal to be transmitted to a terminal which performs a CoMP operation in a multi-cell environment is a cell-specific reference signal or a CoMP-zone-specific reference signal. A method for transmitting the cell-specific reference signal uses an ID of a CoMP set constituted by cells that perform a CoMP operation, or does not apply a frequency shift value while using the ID of a relevant cell, or uses an ID of a serving cell or a frequency shift value of the serving cell. The cell-specific reference signal is transmitted using a preset specific resource region. A method for transmitting the CoMP-zone-specific reference signal is configured such that the pattern of the sequence of CoMP reference signals varies in each of the CoMP zones, and the pattern can be set in advance. In addition, the reference signal is multiplexed and transmitted using an orthogonal code resource in the event that reference signal is transmitted to a plurality of terminals using the same CoMP zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/386,384, filed Jan. 20, 2012, which is a 35 U.S.C. §371 NationalStage Entry of International Application No. PCT/KR2010/004880 filedJul. 26, 2010, and claims the benefit of priority of U.S. ProvisionalApplication No. 61/228,164 filed on Jul. 24, 2009, and Korean PatentApplication No. 10-2009-0121788 filed on Dec. 9, 2009, all of which areincorporated by reference in their entirety herein.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method of transmitting and receiving CoMPreference signal in a wireless communication system.

BACKGROUND ART

Recently, MIMO (multiple input multiple output) system is one of thehottest segments in the wideband wireless communication technology. TheMIMO system means the system that can raise communication efficiency ofdata using multiple antennas. And, the MIMO systems can be implementedusing such MIMO scheme as spatial multiplexing scheme and spatialdiversity scheme in accordance with a presence or non-presence of thesame data transmission.

The spatial multiplexing scheme means the scheme for transmitting dataat high speed by transmitting different data via a plurality oftransmitting antennas simultaneously without increasing a bandwidth. Thespatial diversity scheme means the scheme for obtaining transmissiondiversity by transmitting the same data via a plurality of transmittingantennas. Space time channel coding is an example for the spacediversity scheme.

Moreover, the MIMO technique can be categorized into an open loop schemeand a closed loop scheme in accordance with a presence or non-presenceof feedback of channel information to a transmitting side from areceiving side. The open loop scheme may include one of BLAST scheme ofdetecting a signal and increasing an information size amounting to thenumber of transmitting antennas in a manner that a transmitting sidetransmits information in parallel and that a receiving side detects asignal using ZF (zero forcing) and MMSE (minimum mean square error)scheme repeatedly, STTC (space-time trellis code) scheme of obtaining atransmission diversity and coding gain using a new space region, and thelike. And, the closed loop scheme may include one of TxAA (transmitantenna array) scheme and the like.

In a radio channel configuration, a fading effect is generated in amanner that a channel status irregularly changes in time and frequencydomains. In order to reconstruct data transmitted from a transmitter andfind out a correct signal, a receiver corrects a received signal usingchannel information.

A wireless communication system transmits a signal known to atransmitter and a receiver both and then finds out channel informationusing an extent of the signal that is distorted on being transmitted ona channel. In this case, this signal is called a reference signal (or apilot signal) and ‘finding out channel information’ is called channelestimation. The reference signal does not contain data actually and hashigh output. In case that data is transmitted or received using multipleantennas, it may be necessary to obtain a channel status between eachtransiting antenna and each receiving antenna. Hence, the referencesignal exists for each transmitting antenna.

Coordinated MIMO system is proposed to reduce inter-cell interference ina multi-cell environment. If the coordinated MIMO system is used, a userequipment may be jointly supported with data in common by multiple basestations (multi-cell base-station). And, in order to improve performanceof system, each base station may be able to support at least one userequipments (MS1, MS2, . . . , MSK) simultaneously using the samefrequency resource. Moreover, the base station may be able to performSDMA (space division multiple access) based on channel state informationbetween the base station and the user equipment.

In the coordinated MIMO system, a serving base station and at least oneor more cooperative base stations are connected to a scheduler viabackbone network. The scheduler may be activated in a manner ofreceiving feedback of channel information on channel states between thecooperative base stations and the user equipments (MS1, MS2, . . . ,MSK), which are measured by the base stations (BS1, BS2, . . . BSM) viathe backbone network. For instance, information for the coordinated MIMOoperation is scheduled for the serving base station and the at least oneor more cooperative base stations by the scheduler. In particular, thescheduler directs instruction for the coordinated MIMO operation to eachof the base stations.

CoMP has been proposed to reduce inter-cell interference and improveperformance of a user equipment on a cell boundary in a multi-cellenvironment. Using CoMP system, a user equipment may be supported withdata in common by a multi-cell base station. In particular, in amulti-cell environment, it may be able to improve communicationperformance of a user equipment using CoMP scheme. For this, accuratechannel estimation is necessary based on a reference signal from themulti-cell base station.

Basically, cells of the related art generate reference signal sequencesand patterns based on cell identifiers (IDs), respectively. However, incase of a coherent joint processing scheme capable of transmitting datasimultaneously from multi-cell, this reference signal generation maycause degradation of communication performance.

DISCLOSURE OF THE INVENTION Technical Tasks

An object of the present invention is to provide a method oftransmitting and receiving CoMP reference signal.

Another object of the present invention is to provide a base stationapparatus for transmitting CoMP reference signal.

A further object of the present invention is to provide a user equipmentapparatus for receiving CoMP reference signal.

Technical tasks obtainable from the present invention may be non-limitedby the above mentioned technical tasks. And, other unmentioned technicaltasks can be clearly understood from the following description by thosehaving ordinary skill in the technical field to which the presentinvention pertains.

Technical Solution

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a method oftransmitting a CoMP (cooperative multi-point) reference signal at a basestation, according to the present invention includes the steps ofgenerating a CoMP reference signal sequence corresponding to a CoMPscheme of a CoMP set including cells operating by a CoMP scheme havingthe base station belong thereto, mapping the generated CoMP referencesignal sequence to a specific resource region for the CoMP scheme, andtransmitting the CoMP reference signal mapped to the specific resourceregion to a user equipment.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a method of receiving a CoMP(cooperative multi-point) reference signal at a user equipment,according to the present invention includes the steps of receiving theCoMP reference signal corresponding to a specific CoMP scheme from eachcell performing the CoMP scheme and identifiably processing the receivedCoMP reference signal per the each cell performing the CoMP operation orper CoMP set of cells performing the CoMP operation, wherein the CoMPreference signal is mapped to a specific resource region for thespecific CoMP scheme.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a method of transmitting a CoMP(cooperative multi-point) reference signal at a base station, accordingto the present invention includes the steps of allocating a specificresource region to transmit the CoMP reference signal, generating a CoMPreference signal sequence of a pattern equal to a CoMP reference signalsequence pattern previously set for the allocated resource region,mapping the generated CoMP reference signal sequence to the allocatedresource region, and transmitting the mapped CoMP reference signalsequence to a user equipment.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a base station apparatus fortransmitting a CoMP (cooperative multi-point) reference signal accordingto the present invention includes a sequence generating modulegenerating a CoMP reference signal sequence corresponding to a CoMPscheme of a CoMP set of cells operating by a CoMP scheme having the basestation apparatus belong thereto, a resource mapper mapping thegenerated CoMP reference signal sequence to a specific resource regionfor the CoMP scheme, and a transmitter transmitting the CoMP referencesignal mapped to the specific resource region to a user equipment.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a user equipment apparatus forreceiving a CoMP (cooperative multi-point) reference signal according tothe present invention includes a receiver receiving the CoMP referencesignal corresponding to a specific CoMP scheme from each cell performingthe CoMP scheme and a processor identifiably processing the receivedCoMP reference signal per the each cell performing the CoMP operation orper CoMP set of cells performing the CoMP operation, wherein the CoMPreference signal is mapped to a specific resource region for thespecific CoMP scheme.

Advantageous Effects

In case that a user equipment receives CoMP reference signal of thepresent invention, influence of inter-cell interference is considerablyreduced and accurate channel estimation from multiple base stations ispossible.

Effects obtainable from the present invention may be non-limited by theabove mentioned effect. And, other unmentioned effects can be clearlyunderstood from the following description by those having ordinary skillin the technical field to which the present invention pertains.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram for explaining physical channels used for such amobile communication system as 3GPP (3^(rd) generation partnershipproject) LTE (long term evolution) system and a general signaltransmitting method using the physical channels;

FIG. 2 is a diagram for describing a signal processing process for abase station to transmit a downlink signal in 3GPP LTE system forexample of a mobile communication system;

FIG. 3 is a diagram of a time-frequency resource grid structure used bythe present invention;

FIGS. 4( a) and 4(b) are diagrams illustrating for one example of aspecific resource region for generation and transmission of CoMPreference signal; and

FIG. 5 is a block diagram for configurations of a base station t05 and auser equipment 510 in a wireless communication system according to thepresent invention.

BEST MODE FOR INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. In the following detailed description of the inventionincludes details to help the full understanding of the presentinvention. Yet, it is apparent to those skilled in the art that thepresent invention can be implemented without these details. Forinstance, although the following descriptions are made in detail on theassumption that a mobile communication system includes 3GPP LTE system,the following descriptions are applicable to other random mobilecommunication systems in a manner of excluding unique features of the3GPP LTE.

Occasionally, to prevent the present invention from getting vaguer,structures and/or devices known to the public are skipped or can berepresented as block diagrams centering on the core functions of thestructures and/or devices. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Besides, in the following description, assume that a terminal is acommon name of such a mobile or fixed user stage device as a userequipment (UE), a mobile station (MS), an advanced mobile station (AMS)and the like. And, assume that a base station (BS) is a common name ofsuch a random node of a network stage communicating with a terminal as aNode B (NB), an eNode B (eNB), an access point (AP) and the like.

In a mobile communication system, a user equipment is able to receiveinformation in downlink and is able to transmit information in uplink aswell. Informations transmitted or received by the user equipment mayinclude various kinds of data and control informations. In accordancewith types and usages of the informations transmitted or received by theuser equipment, various physical channels may exist.

FIG. 1 is a diagram for explaining physical channels used for such amobile communication system as 3GPP (3^(rd) generation partnershipproject) LTE (long term evolution) system and a general signaltransmitting method using the physical channels.

Referring to FIG. 1, a user equipment performs initial cell search suchas synchronizing with a base station when it newly enters a cell or thepower is turned on again (S101). To this end, the user equipmentsynchronizes with the base station by receiving a primarysynchronization channel (P-SCH) and a secondary synchronization channel(S-SCH) from the base station, and acquires information of cell ID, etc.Afterwards, the user equipment can acquire broadcast information withinthe cell by receiving a physical broadcast channel from the basestation. Meanwhile, the user equipment receives a downlink referencesignal in the initial cell searching step and may be then able to checka downlink channel state.

Having finished the initial cell search, the user equipment may acquirefurther detailed system information by receiving a physical downlinkcontrol channel (PDCCH) and a physical downlink shared channel (PDSCH)in accordance with the physical downlink control channel information(S102).

Meanwhile, if the user equipment initially accesses the base station orthere is no radio resource for signal transmission, the user equipmentmay perform a random access procedure on the base station (S103 toS106). To this end, the user equipment transmits a specific sequence asa preamble on a physical random access channel (PRACH) (S103) and maythen receive a response message to the random access through the PDCCHand the PDSCH corresponding to the PDCCH (S104). In case of a contentionbased random access except a case of handover, such a contentionresolution procedure as a physical random access channel transmissionS105 and a PDCCH/PDSCH reception S106 may be performed additionally.

Having performed the aforementioned steps, the user equipment mayperform such a general UL/DL signal transmitting step as a PDCCH/PDSCHreception (S107) and a PUSCH/PUCCH (physical uplink sharedchannel/physical uplink control channel) transmission (S108). In thiscase, the control information transmitted from the user equipment to thebase station or received from the base station by the user equipment inuplink may include DL/UL (downlink/uplink) ACK/NACK signals, a channelquality indicator (hereinafter abbreviated CQI), a precoding matrixindex (hereinafter abbreviated PMI), a rank indicator (hereinafterabbreviated RI) and the like. In case of the 3GPP LTE (3^(rd) generationpartnership project long term evolution) system, the user equipment cantransmit control information such CQI, PMI, RI and the like on the PUSCHand/or the PUCCH.

FIG. 2 is a diagram for describing a signal processing process for abase station to transmit a downlink signal in 3GPP LTE system forexample of a mobile communication system.

In 3GPP LTE system, a base station may be able to transmit at least onecodewords in downlink. Hence, the at least one codeword may be processedinto a complex symbol via a scrambling module 201 and a modulationmapper 202. Thereafter, the complex symbol is mapped to a plurality oflayers by a layer mapper 203. And, each of the layers may be allocatedto each transmitting antenna by being multiplied by a prescribedprecoding matrix selected by a precoding module in accordance with achannel state. This processes transmitted signal per antenna is mappedto a time-frequency resource element by a resource element mapper 205 tobe used for transmission and may be then transmitted via an OFDM signalgenerator 206 and a corresponding antenna.

FIG. 3 is a diagram of a downlink time-frequency resource grid structureused by the present invention.

A downlink (hereinafter abbreviated DL) signal transmitted in each slotuses a resource grid structure constructed with N_(RB) ^(DL)×N_(SC)^(RB) subcarriers and N_(symb) ^(DL) OFDM (Orthogonal Frequency DivisionMultiplexing) symbols. In this case, ‘N_(RB) ^(DL)’ indicates the numberof resource blocks (RBs) in DL, ‘N_(SC) ^(RB)’ indicates the number ofsubcarriers constructing one RB, and ‘N_(symb) ^(DL)’ indicates thenumber of OFDM symbols in one DL slot. A size of ‘N_(RB) ^(DL)’ variesin accordance with a DL transmission bandwidth configured within a celland should meet ‘N_(RB) ^(min,DL)≦N_(RB) ^(DL)≦N_(RB) ^(max,DL)’. Inthis case, RB ‘N_(RB) ^(min,DL)’ is a smallest DL bandwidth supported bya wireless communication system and ‘N_(RB) ^(max,DL)’ is a greatest DLbandwidth supported by the wireless communication system. It may become‘N_(RB) ^(min,DL)’ and ‘N_(RB) ^(max,DL)=110’, by which the presentexample is non-limited. The number of the OFDM symbols included in oneslot can vary in accordance with a length of a CP (cyclic prefix) and aninterval of subcarrier. In caser of multi-antennal transmission, oneresource grid can be defined for each antenna port.

Each element within the resource grid for each antenna port is called aresource element (hereinafter abbreviated RE) and is uniquely identifiedby an index pair (k, 1) within a slot. In this case, ‘k’ is an index ina frequency domain and ‘1’ is an index in a time domain. The ‘k’ has avalue selected from ‘0, . . . , N_(RB) ^(DL)N_(SC) ^(RB)−1’ and the ‘1’has a value selected from ‘0, . . . , N^(DL) _(symb)−1’.

The resource block shown in FIG. 3 is used to describe the mappingrelation between a prescribed physical channel and resource elements.Resource blocks can be classified into physical resource blocks (PRBs)and virtual resource blocks (VRBs). One PRB can be defined by N_(symb)^(DL) contiguous OFDM symbols in time domain and N_(SC) ^(RB) contiguoussubcarriers in frequency domain. In this case, ‘N_(symb) ^(DL)’ and‘N_(SC) ^(RB)’ can be given as shown in Table 3. Hence, one PRB isconstructed with ‘N_(symb) ^(DL)×N_(SC) ^(RB)’ resource elements. OnePRB corresponds to one slot in time domain and also corresponds to 180kHz in frequency domain, by which the present example is non-limited.

TABLE 1 Configuration N_(sc) ^(RB) N_(symb) ^(DL) Normal Δf = 15 kH 12 7cyclic prefix Extended Δf = 15 kH 6 cyclic prefix Δf = 7.5 k

24 3

indicates data missing or illegible when filed

The terminology ‘base station’ used in the present invention isconceptionally used to include a cell or a sector. In case of being usedas a regional concept, the terminology ‘base station’ may be named acell or a sector. A serving base station (or cell) may be regarded as abase station (or cell) that provides a major service to a user equipmentand may be able to perform transmission/reception of control informationat a coordinated multiple transmission point. In this meaning, a servingbase station (or cell) may be named an anchor base station (or anchorcell). A serving base station may be able to transmit various kinds ofinformations received from a user equipment to a neighbor base station(or cell). Likewise, in case that a neighbor base station is used as aregional concept, it may be called a neighbor cell. In the presentinvention, one CoMP set means a set of cells capable of performing CoMPoperation. Cell belonging to CoMP set may switch its operation to enternon-CoMP mode in the course of performing CoMP operation.

If CoMP scheme is used in a multi-cell environment, it may be able toenhance communication performance of a user equipment on a cellboundary. This CoMP scheme may be categorized into JP (joint processing)of a coordinated MIMO type through data sharing, CS/CB (coordinatedscheduling/beamforming) for reducing inter-cell interference like worstcompanion or best companion, geographically remote transport process(e.g., multi-antenna) scheme and the like.

Specifically, the CS/CB (coordinated scheduling/beamforming) scheme isthe method of reducing inter-cell interference and may be able to reduceinterference from a neighbor cell in a manner that a user equipmenttransmits a limited and/or recommended PMI to a serving base station. Inthis case, the worst companion scheme is a method for eliminatinginter-cell interference in a manner that a user equipment reports PMIhaving a biggest interference with cells for CoMP operating cells to aserving base station and that the corresponding neighbor cells use asecond-best PMI except the corresponding PMI. On the contrary, the bestcompanion scheme is a method for reducing inter-cell interference in amanner that a user equipment reports PMI having a smallest interferencewith cells for CoMP operating cells and that the corresponding neighborcells use the corresponding PMI.

Using the above-configured CoMP system, a user equipment may besupported with data jointly from multiple base stations (multi-cell basestation). In particular, it may be able to improve communicationperformance of a user equipment on a cell boundary using the CoMPoperation in a multi-cell environment.

Details of CoMP reference signal for LTE-A (long termevolution-advanced) system have not been defined yet. In general,reference signals for performing CoMP include a common reference signal(CRS) for the usage of channel state measurement of channel stateinformation of multi-cell and the like and a demodulation referencesignal (DRS) for the usage of demodulation.

DRS sequence usable for CoMP reference signal may be mapped within oneresource block unlike CRS. Compared to CRS mapped across a whole regionon a frequency axis, DRS may map a reference signal sequence by unit ofphysical resource block (PRB). For instance, DRS sequence having alength of 12 may be mapped within one resource block.

According to the present invention, first of all, a reference signaltransmitting method using DRS in joint processing (JP) scheme among CoMPschemes in multi-cell environment is described. In particular, in orderfor multi-cell to transmit a desirable signal via a frequency resourceregion, the joint processing scheme using RF combining (or coherent) isdescribed.

If multiple cells, which perform CoMP operation, transmit desirablesignals to a user equipment located on a cell boundary of a serving cellusing the same resource region (time/frequency region), it may be calleda coherent or RF combining scheme. In order to use this RF combiningscheme, it may be able to apply inter-cell MIMO scheme throughinter-cell cooperation. This MIMO scheme may include one of transmitdiversity (TxD) scheme such as single frequency network (SFN)transmission or space time block code (SFBC) for transmitting the samedata between cells, spatial multiplexing (SM) scheme of higher layer,and the like.

In case that data is transmitted using a reference signal based on cellID of cells differing from each other in coherent MIMO transmissionbetween multi-cells, communication performance may be lowered. Dataresource element and reference signal may collide with each other due todifferent reference signal sequences or patterns between multiple cells,which cause degradation of channel estimation performance of a userequipment. Moreover, different reference signals between multiple cellsin applying such transmit diversity scheme as time space block code(SFBC) may break paring between data to obtain diversity gain.

In order to solve the problem of the coherent joint processing schemeaccording to using different reference signals between multiple cells,it may be able to maintain the same reference signal sequence andpattern of cells belonging to CoMP set that performs the coherent jointprocessing scheme. In particular, if multiple cells, which perform theCoMP operation, transmit the same reference signal sequence based on thesame cell ID using the same resource region (time/frequency region), itmay be able to basically solve the above problem.

In the following description, various embodiments of a method formultiple cells to generate CoMP reference signal sequence are described.

In a method of generating a CoMP reference signal sequence according toone embodiment of the present invention, assuming that a referencesignal sequence r(m) is mapped to a complex value modulation symbola_(k,l) ^((p)) using antenna part 5, it may be able to generate a CoMPreference signal sequence using Formula 1.

$\begin{matrix}{{a_{k,l}^{(p)} = {r\left( {{3 \cdot l^{\prime} \cdot N_{RB}^{PDSCH}} + m^{\prime}} \right)}}{k = {{\left( k^{\prime} \right){mod}\; N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}}{k^{\prime} = \left\{ {{\begin{matrix}{4\; m^{\prime}} & {{{if}\mspace{14mu} l} \in \left\{ {2,3} \right\}} \\{{4\; m^{\prime}} + {2\; {mod}\; 4}} & {{{if}\mspace{14mu} l} \in \left\{ {5,6} \right\}}\end{matrix}l} = \left\{ {{\begin{matrix}3 & {l^{\prime} = 0} \\6 & {l^{\prime} = 1} \\2 & {l^{\prime} = 2} \\5 & {l^{\prime} = 3}\end{matrix}l^{\prime}} = \left\{ {{{\begin{matrix}{0,1} & {{{if}\mspace{14mu} n_{s}{mod}\; 2} = 0} \\{2,3} & {{{if}\mspace{14mu} n_{s}{mod}\; 2} = 1}\end{matrix}m^{\prime}} = 0},1,\ldots \mspace{14mu},{{3\; N_{RB}^{PDSCH}} - 1}} \right.} \right.} \right.}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In this case, except a cell-specific frequency shift equation‘v_(shift)=N_(ID) ^(cell) mod 3’, a CoMP reference signal sequence isgenerated using Formula 1. A neighbor cell, which performs CoMPoperation (e.g., coherent joint processing), generates a referencesignal sequence based on DRS sequence and pattern of a serving cell andmay be then able to transmit the generated reference signal sequence ofthe same pattern to a user equipment.

For example of a method of generating CoMP reference signal sequenceaccording to one embodiment of the present invention, a CoMP set ID(N_(ID) ^(CoMPset)) is set in advance and a reference signal sequence isthen generated using Formula 2 based on the same CoMP set ID among CoMPset IDs previously set by a serving cell and neighbor cells, whichperform CoMP.

$\begin{matrix}{{k = {{\left( k^{\prime} \right){mod}\; N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}}{k^{\prime} = \left\{ {{\begin{matrix}{{4\; m^{\prime}} + v_{CoMP\_ shift}} & {{{if}\mspace{14mu} l} \in \left\{ {2,3} \right\}} \\{{4\; m^{\prime}} + {\left( {2 + v_{CoMP\_ shift}} \right){mod}\; 4}} & {{{if}\mspace{14mu} l} \in \left\{ {5,6} \right\}}\end{matrix}l} = \left\{ {{\begin{matrix}3 & {l^{\prime} = 0} \\6 & {l^{\prime} = 1} \\2 & {l^{\prime} = 2} \\5 & {l^{\prime} = 3}\end{matrix}l^{\prime}} = \left\{ {{{\begin{matrix}{0,1} & {{{if}\mspace{14mu} n_{s}{mod}\; 2} = 0} \\{2,3} & {{{if}\mspace{14mu} n_{s}{mod}\; 2} = 1}\end{matrix}m^{\prime}} = 0},1,\ldots \mspace{14mu},{{3\; N_{RB}^{PDSCH}} - 1}} \right.} \right.} \right.}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Neighbor cells may be able to transmit the reference signal sequence ofthe same pattern generated using Formula 2. In Formula 2, using the CoMPset-specific frequency shift ‘v_(CoMP) _(—) _(shift)=N_(ID) ^(CoMP set)mod N’, it may be able to generate the same DRS sequence and pattern ofcells belonging to a specific CoMP set.

As mentioned in the foregoing description, in order to smoothly performthe multi-cell based coherent joint processing scheme, a referencesignal sequence between cells performing the CoMP operation isidentically generated and may be then transmitted in the same pattern toa user equipment. In order to minimize the influence on a legacy userequipment by maintaining the advantage of the coherent joint processingscheme as maximal as possible, reference signal adaptation between CoMPscheme and non-CoMP scheme or between CoMP schemes is necessary. A cellbelonging to one CoMP set in the course of performing an operation by aspecific CoMP scheme may be able to switch it operation to enter adifferent CoMP scheme mode or a non-CoMP scheme mode.

In case that CoMP performing cells perform the coherent joint processingscheme, as mentioned in the foregoing description, the same referencesignal sequence on the basis of the same ID is generated and thentransmitted in the same pattern to a user equipment. On the other hand,if neighbor cells operate by the cooperative scheduling/beamforming(CS/CB) scheme or the non-CoMP scheme, it may be able to generate areference signal sequence using Formula 3.

$\begin{matrix}{{k = {{\left( k^{\prime} \right){mod}\; N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}}{k^{\prime} = \left\{ {{\begin{matrix}{{4\; m^{\prime}} + v_{shift}} & {{{if}\mspace{14mu} l} \in \left\{ {2,3} \right\}} \\{{4\; m^{\prime}} + {\left( {2 + v_{shift}} \right){mod}\; 4}} & {{{if}\mspace{14mu} l} \in \left\{ {5,6} \right\}}\end{matrix}l} = \left\{ {{\begin{matrix}3 & {l^{\prime} = 0} \\6 & {l^{\prime} = 1} \\2 & {l^{\prime} = 2} \\5 & {l^{\prime} = 3}\end{matrix}l^{\prime}} = \left\{ {{{\begin{matrix}{0,1} & {{{if}\mspace{14mu} n_{s}{mod}\; 2} = 0} \\{2,3} & {{{if}\mspace{14mu} n_{s}{mod}\; 2} = 1}\end{matrix}m^{\prime}} = 0},1,\ldots \mspace{14mu},{{3\; N_{RB}^{PDSCH}} - 1}} \right.} \right.} \right.}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In order to perform CoMP operation efficiently and reduce feedback andscheduling overhead, it may be able to set a specific resource region(called ‘CoMP’ zone) for a specific CoMP scheme among resource regions.By setting a physical resource block (PRB) for CoMP operation, a CoMPperforming user equipment may be able to perform measurement andfeedback on CoMP zone. In accordance with this CoMP zone setting, it maybe able to considerably reduce scheduling information between CoMPoperation performing cells. This CoMP zone may be specifically furtheruseful in case of a specific CoMP scheme (e.g., joint processing scheme)for CoMP cells to transmit a desirable signal to a user equipment ingeneral.

Cells, which perform the coherent joint processing scheme, may be ableto set the same CoMP zone on the same resource region (time/frequencyregion) for a user equipment located on a cell boundary. Thus, each ofthe cells, which perform the coherent joint processing scheme, generatesa reference signal by one of the aforesaid CoMP reference signalgenerating methods using the previously set CoMP zone and may be thenable to transmit the generated reference signal of the same pattern to auser equipment. And, a reference signal sequence is generated in therest of the resource region except the CoMP zone based on a unique cellID of each of the cells and may be then transmitted to the userequipment.

It may be necessary to define signaling to adaptively perform the modeswitching between the above-described CoMP schemes or the mode switchingbetween a specific CoMP scheme and a non-CoMP scheme.

First of all, cells, which perform the coherent joint processing scheme,may be able to generate the same reference signal using Formula 1 byexcluding a different cell-specific frequency shift formula‘v_(shift)=N_(ID) ^(cell) mod 3’ In this case, a serving cell, to whicha user equipment located on a cell boundary belongs, may be able to turnoff the generation of V_(shift) value from a reference signal sequencecurrently generated by a corresponding cell ID by delivering anindication of the coherent joint processing scheme mode to neighborcells that perform the CoMP operation. In doing so, in case that theCoMP performing cells transmit CoMP reference signal to perform thecoherent joint processing scheme among the CoMP schemes using CoMP zoneof a specific resource region specifically, the serving cell may be ableto inform a user equipment and/or a neighbor cell of information on theCoMP zone region.

On the other hand, in order for neighbor cells to generate CoMPreference signal based on a cell ID of a serving cell and transmit thegenerated CoMP reference signal of the same pattern to a user equipment,the serving cell may deliver its cell ID information to a neighbor cell.In doing so, this indication information may be delivered via x2interface, backhaul or the like. Alternatively, CoMP performing cellsmay be able to generate CoMP reference signal sequence to correspond toV_(shift) value of the serving cell. In doing so, the CoMP referencesignal sequence may be generated based on each cell ID. Likewise, inthis case, when CoMP reference signal is transmitted using CoMP zone ofa specific resource region specifically to perform the coherent jointprocessing scheme, the serving cell may be able to inform a userequipment and/or a neighbor cell of information on the CoMP zone region.

Meanwhile, in case that a coherent joint processing scheme is performedin a manner of setting CoMP set ID in advance, a user equipment orneighbor cells may receive information on a start of the coherent jointprocessing scheme, CoMP set ID information and the like from a servingcell. Moreover, when CoMP reference signal is transmitted using CoMPzone of a specific resource region specifically to perform the coherentjoint processing scheme, the serving cell may be able to inform a userequipment and/or a neighbor cell of information on the CoMP zone regionin advance.

Let's consider a situation that a plurality of user equipmentssimultaneously perform CoMP operation within one CoMP set among CoMPoperations.

First of all, in case that multi-cells perform CoMP operation oftransmitting a desirable signal for one user equipment only (e.g.,multi-cell single user-MIMO (multi-cell SU-MIMO)), as mentioned in theforegoing description, in order for the multi-cells belonging thereto togenerate the same CoMP reference signal, 1) a serving cell controlsneighbor cells to turn off the generation of a frequency shift valueV_(shift), or 2) delivers its cell ID information to a neighbor cell toenable neighbor cells to generate CoMP reference signal sequence basedon cell ID of the serving cell. Alternatively, 3) the serving cellcontrols CoMP performing cells to generate the same CoMP referencesignal based on CoMP set ID previously defined to perform the coherentjoint processing scheme and then to transmit the generated CoMPreference signal of the same pattern.

From now on, consider a situation that one base station performs acoherent joint processing scheme for a plurality of user equipments.

FIG. 4 is a diagram for one example of a specific resource region forgeneration and transmission of CoMP reference signal.

In case that multi-cells perform CoMP operation to transmit a desirablesignal to at least one user equipment (e.g., multi-cell multi-user-MIMO(multi-cell MU-MIMO)), each user equipment may receive allocation of adifferent specific resource region (e.g., 1^(st) CoMP zone, 2^(nd) CoMPzone, etc.) or may be multiplexed with the same CoMP zone.

Referring to FIG. 4 (a), in case that each user equipment receivesallocation of a different CoMP zone, each of multiple cells performingCoMP operation turns off V_(shift) of each CoMP zone and then transmitsCoMP reference signal, thereby efficiently performing the coherent jointprocessing scheme on the user equipment belonging to the correspondingCoMP zone. On the other hand, multi-cells with reference to specificcell ID among multi-cells in the course of performing CoMP operationgenerate the same CoMP reference signal and then transmit the generatedsame CoMP reference signal to a user equipment. Alternatively, in CoMPzone to which a corresponding user equipment belongs with reference tocell ID of a serving cell to which each user equipment belongs, allcells performing CoMP operation generate the same CoMP reference signaland may be then able to transmit the generated same CoMP referencesignal to the user equipment. Besides, after CoMP set ID has beendefined in advance, all cells performing CoMP operation generate andtransmit the same CoMP reference signal to a user equipment based on thedefined CoMP set ID.

Unlike the above-mentioned method of generating and transmitting thecell-specific CoMP reference signal sequence, CoMP zone-specific methodof respectively generating and transmitting different CoMP referencesignals for CoMP zones is proposed as follows.

First of all, multi-cells may be able to transmit CoMP reference signalbased on CoMP reference signal sequence and pattern previously definedper CoMP zone currently serviced by the corresponding multi-cell. Aserving cell having a user equipment belong thereto may inform anotherneighbor cell of CoMP reference signal information on the correspondingCoMP zone. Alternatively, after CoMP zone ID different per resourceregion corresponding to CoMP zone has been set in advance, each basestation may generate and transmit a previously-agreed CoMP referencesignal sequence according to the CoMP zone ID.

Referring to FIG. 4 (b), if each user equipment is multiplexed with thesame CoMP zone, each cell performing CoMP operation may be able totransmit CoMP reference signal using the same CoMP zone based onorthogonal code resource. In this case, the code resource may includeone of all orthogonal codes such as Walsh-Hadamard code, DFT (discreteFourier transform) orthogonal code (circular shift) and the like. Eachcell may deliver information indicating that CoMP reference signal iscurrently transmitted on a specific orthogonal code resource to eachuser equipment within the same CoMP zone. A basic CoMP reference signalsequence is generated based on the above-mentioned methods, and anorthogonal code resource may be usable for another user equipment withinthe same CoMP zone in addition.

Information (e.g., coherent joint processing scheme mode startinformation, CoMP set ID index, information on orthogonal code resource,etc.) for the aforesaid coherent joint processing scheme supposed to bedelivered to a user equipment from a serving cell or the like may benamed ‘coherent joint processing scheme information’. This coherentjoint processing scheme information may be delivered by higher layersignaling or L1/L2 control signaling.

A serving base station may be able to inform a user equipment of‘coherent joint processing scheme information’. This information may betransmitted to the user equipment periodically or at an event-triggeredtiming point. Meanwhile, the serving base station may be able totransmit information on ‘coherent joint processing scheme information’to the user equipment by the L1/L2 control signaling.

Generally, a base station may be able to transmit scheduling allocationinformation and other control informations and the like on PDCCH. Aphysical control channel may be transmitted as one aggregation or aplurality of contiguous control channel elements (CCEs). One CCE mayinclude 9 resource element groups. The number of resource element groupsnot allocated to PCFICH (physical control format indicator channel) orPHICH (physical hybrid automatic repeat request indicator channel) isN_(REG). CCEs available for a system range 0 to N_(CCE)−1, whereN_(CCE)=└N_(REG)/9┘. PDCCH supports such multi-format as shown in Table2. One PDCCH constructed with n contiguous CCEs starts with CCE thatperforms ‘i mod n=0’, where i is CCE index). Multi-PDCCHs may betransmitted in one subframe.

TABLE 2 PDCCH Number of Number of resource- Number of format CCEselement groups PDCCH bits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

Referring to Table 2, a base station may be able to determine a PDCCHformat in accordance with how many regions will be used to carry controlinformation and the like. And, a user equipment may be able to reduceoverhead by reading the control information and the like by CCE unit.PDCCH of a DCI (downlink control information) format type configured ina format according to control information a serving station attempts totransmit may be designed identifiably. In this case, in viewpoint ofreusing a previous DCI format, a DCI format may be configured in amanner of reusing some fields of a random DCI format and padding therest of the fields with zero padding or arbitrary values. And,information on the configured DCI format may be transmitted to a userequipment.

For instance, assume that simple scheduling on rank 1 transmission ofsingle codeword is transmitted in spatial multiplexing mode using DCIformat 1B. Table 3 shows one example of downlink control informationtransmitted using DCI format 1B.

TABLE 3 Information field Bit(s) Localized/distributed VRB 1 assignmentflag Resource block allocation ┌log₂(N_(RB) ^(DL))(N_(RB) ^(DL) + 1)/2┐Modulation & coding scheme (MCS) 5 # of HARQ processing 3 (FDD), 4 (FDD)New data indicator 1 Redundancy version 2 TPC command for PUCCH 2Downlink assignment index 2

Referring to Table 3, DCI format 1B includes a plurality of informationfields. In particular, a plurality of the information fields may includea localized/distributed virtual resource block (VRB) assignment flagfield, a resource block allocation field, a modulation and coding (MCS)scheme field, an HARQ processing number field, a new data indicatorfield, a TCP command field for PUCCH, a downlink assignment index field,a TPMI (transmitted precoding matrix indicator) information field forprecoding, a PMI approval field for precoding, and the like.

This DCI format 1B may be configured as Table 4 to support a jointprocessing scheme among CoMP schemes.

TABLE 4 Information field Bit(s) Localized/distributed VRB 1 assignmentflag Resource block allocation ┌log₂(N_(RB) ^(DL))(N_(RB) ^(DL) + 1)/2┐Modulation & coding scheme (MCS) 5 # of HARQ processing 3 (FDD), 4 (FDD)New data indicator 1 Redundancy version 2 TPC command for PUCCH 2Downlink assignment index 2 Coherent joint processing information 5(e.g., coherent joint processing mode start information, CoMP set IDindex, orthogonal code resource, etc.)

Referring to Table 4, the bit number of each information field is justexemplary, b which a size of the information field may be non-limited.In order to provide commonality of downlink transmission irrespective oftransmission mode, it may be able to transmit information having thesame size of DCI format for single user MIMO (SU-MIMO), multi-user-MIMO(MU-MIMO) and the like.

Since a CoMP performing user equipment is based on a modulationreference signal from multi-cell, it may be able to delete informationbits for PMI. And, ‘coherent joint processing scheme information’ may becarried on a space obtained from the deletion of information bits. Sincethe coherent joint processing scheme information is variable, it may beconfigured in a manner that some fields are used and that the rest ofthe fields are padded with zero padding or arbitrary values.

As mentioned in the above description, a cell-boundary user equipmentconsiderably reduces inter-cell interference in a multi-cell environmentin accordance with CoMP reference signal generating and transmittingmethods according to the present invention and may perform accuratechannel estimation from a multi-cell base station smoothly.

FIG. 5 is a block diagram for configurations of a base station 505 and auser equipment 510 in a wireless communication system according to thepresent invention.

Although one base station 505 and one user equipment 510 are shown inthe drawing to schematically represent a wireless communication system500, the wireless communication system 500 may include at least one basestation and/or at least one user equipment.

Referring to FIG. 5, a base station 505 may include a transmitted (Tx)data processor 515, a symbol modulator 520, a transmitter 525, atransceiving antenna 530, a processor 580, a memory 585, a receiver 590,a symbol demodulator 595 and a received data processor 597. A userequipment 510 may include a transmitted (Tx) data processor 565, asymbol modulator 570, a transmitter 575, a transceiving antenna 535, aprocessor 555, a memory 560, a receiver 540, a symbol demodulator 555and a received data processor 550. Although the base station/userequipment 505/510 is configured to include one antenna 530/535 in thedrawing, each of the base station 505 and the user equipment 510 mayinclude multi-antenna having a plurality of antennas. Therefore, each ofthe base station 505 and the user equipment 510 supports MIMO (multipleinput multiple output) system. And, the base station/user equipment505/510 according to the present invention may support both SU-MIMO(single user-MIMO) and MU-MIMO (multi user-MIMO).

In downlink, the transmitted data processor 515 receives traffic data,codes the received traffic data by formatting the received traffic data,interleaves the coded traffic data, modulates (or symbol maps) theinterleaved data, and then provides modulated symbols (data symbols).The symbol modulator 520 provides a stream of symbols by receiving andprocessing the data symbols and pilot symbols.

The symbol modulator 520 multiplexes the data and pilot symbols togetherand then transmits the multiplexed symbols to the transmitter 525. Indoing so, each of the transmitted symbols may include the data symbol,the pilot symbol or a signal value of zero. In each symbol duration,pilot symbols may be contiguously transmitted. In doing so, the pilotsymbols may include symbols of frequency division multiplexing (FDM),orthogonal frequency division multiplexing (OFDM), or code divisionmultiplexing (CDM).

The transmitter 525 receives the stream of the symbols, converts thereceived stream to at least one or more analog signals, additionallyadjusts the analog signals (e.g., amplification, filtering, frequencyupconverting), and generates a downlink signal suitable for atransmission on a radio channel. Subsequently, the downlink signal istransmitted to the user equipment via the antenna 530.

The processor 580 of the base station 505 according to the presentinvention may include a sequence generating module (not shown in thedrawing) and a resource mapper (not shown in the drawing). The sequencegenerating module may be able to generate CoMP reference signal sequencecorresponding to CoMP scheme of CoMP set that is a set of cellsoperating by CoMP scheme to which the base station belongs to. Theresource mapper may be able to map the generated CoMP reference signalsequence to a specific resource region for the CoMP scheme. And, thetransmitter 525 may be able to transmit the CoMP reference signal mappedto the specific resource region to the user equipment.

In the configuration of the user equipment 510, the antenna 535 receivesthe downlink signal from the base station and then provides the receivedsignal to the receiver 540. The receiver 540 adjusts the received signal(e.g., filtering, amplification and frequency downconverting), digitizesthe adjusted signal, and then obtains samples. The symbol demodulator545 demodulates the received pilot symbols and then provides them to theprocessor 555 for channel estimation.

The symbol demodulator 545 receives a frequency response estimated valuefor downlink from the processor 555, performs data demodulation on thereceived data symbols, obtains data symbol estimated values (i.e.,estimated values of the transmitted data symbols), and then provides thedata symbols estimated values to the received (Rx) data processor 550.The received data processor 550 reconstructs the transmitted trafficdata by performing demodulation (i.e., symbol demapping, deinterleavingand decoding) on the data symbol estimated values.

The processing by the symbol demodulator 545 and the processing by thereceived data processor 550 are complementary to the processing by thesymbol modulator 520 and the processing by the transmitted dataprocessor 515 in the base station 505, respectively.

In the user equipment 510 in uplink, the transmitted data processor 565processes the traffic data and then provides data symbols. The symbolmodulator 570 receives the data symbols, multiplexes the received datasymbols, performs modulation on the multiplexed symbols, and thenprovides a stream of the symbols to the transmitter 575. The transmitter575 receives the stream of the symbols, processes the received stream,and generates an uplink signal. This uplink signal is then transmittedto the base station 505 via the antenna 135.

The receiver 540 of the user equipment 510 according to the presentinvention may be able to receive CoMP reference signal corresponding tothe operating specific CoMP scheme from each cell that performs CoMPoperation. The processor 555 of the user equipment 510 processes theCoMP reference signal received by the receiver 540 in a manner ofsorting the received CoMP signal by each cell performing the CoMPoperation or each CoMP set that is a set of the cells performing theCoMP operation. In doing so, the CoMP reference signal may be mapped tothe specific resource region for the specific CoMP scheme.

In the base station 505, the uplink signal is received from the userequipment 510 via the antenna 530. The receiver 590 processes thereceived uplink signal and then obtains samples. Subsequently, thesymbol demodulator 595 processes the samples and then provides pilotsymbols received in uplink and a data symbol estimated value. Thereceived data processor 597 processes the data symbol estimated valueand then reconstructs the traffic data transmitted from the userequipment 510.

The processor 555/580 of the user equipment/base station 510/505 directsoperations (e.g., control, adjustment, management, etc.) of the userequipment/base station 510/505. The processor 555/580 may be connectedto the memory unit 560/585 configured to store program codes and data.The memory 560/585 is connected to the processor 555/580 to storeoperating systems, applications and general files.

The processor 555/580 may be called one of a controller, amicrocontroller, a microprocessor, a microcomputer and the like. And,the processor 555/580 may be implemented using hardware, firmware,software and/or any combinations thereof. In the implementation byhardware, the processor 555/580 may be provided with one of ASICs(application specific integrated circuits), DSPs (digital signalprocessors), DSPDs (digital signal processing devices), PLDs(programmable logic devices), FPGAs (field programmable gate arrays),and the like. In case of the implementation by firmware or software,firmware or software can be configured to include modules, procedures,and/or functions for performing the above-explained functions oroperations of the present invention. And, the firmware or softwareconfigured to implement the present invention is loaded in the processor555/580 or saved in the memory 560/585 to be driven by the processor555/580.

Layers of a radio protocol between a user equipment and a base stationmay be classified into 1^(st) layer L1, 2^(nd) layer L2 and 3^(rd) layerL3 based on 3 lower layers of OSI (open system interconnection) modelwell known to communication systems. A physical layer belongs to the1^(st) layer and provides an information transfer service via a physicalchannel. RRC (radio resource control) layer belongs to the 3^(rd) layerand provides control radio resourced between UE and network. A userequipment and a base station may be able to exchange RRC messages witheach other via radio communication layer and RRC layers.

As mentioned in the foregoing description, the detailed descriptions forthe preferred embodiments of the present invention are provided to beimplemented by those skilled in the art. While the present invention hasbeen described and illustrated herein with reference to the preferredembodiments thereof, it will be apparent to those skilled in the artthat various modifications and variations can be made therein withoutdeparting from the spirit and scope of the invention. Thus, it isintended that the present invention covers the modifications andvariations of this invention that come within the scope of the appendedclaims and their equivalents. For instance, the respectiveconfigurations disclosed in the aforesaid embodiments of the presentinvention can be used by those skilled in the art in a manner of beingcombined with one another.

Therefore, the present invention is non-limited by the embodimentsdisclosed herein but intends to give a broadest scope matching theprinciples and new features disclosed herein.

INDUSTRIAL APPLICABILITY

Accordingly, an apparatus for transmitting and receiving CoMP referencesignal are applicable to such a mobile communication system as 3GPP LTE,3GPP LTE-A, IEEE 802 and the like.

What is claimed:
 1. A method of generating reference signals by a basestation in a wireless communication system, the method comprising:generating a sequence of a first reference signal using a physical cellidentifier of the base station; transmitting, to a first user equipment(UE), a message including mode information indicating a first mode for acoordinated multi-point (CoMP) operation and a predetermined identifierbeing different from the physical cell identifier of the base station;and generating a sequence of a second reference signal for the first UEusing the predetermined identifier instead of the physical cellidentifier of the base station.
 2. The method of claim 1, wherein themessage is transmitted via radio resource reconfiguration (RRC)signaling.
 3. The method of claim 1, wherein the first reference signalis a cell-specific reference signal and the second reference signal is aUE-specific reference signal.
 4. The method of claim 1, furthercomprising: transmitting, to the first UE, the sequence of the secondreference signal and a downlink control information (DCI) including afield indicating an index of the predetermined identifier.
 5. The methodof claim 1, further comprising: broadcasting the physical cellidentifier via a primary-synchronization signal (PSS) and asecondary-synchronization signal (SSS).
 6. The method of claim 1,further comprising: generating a sequence of a third reference signalfor a second UE in a second mode for a non-CoMP operation using thephysical cell identifier of the base station.
 7. The method of claim 1,further comprising: mapping the sequence of the second reference signalon resource elements without physical cell identifier basedcell-specific frequency shifting (V_(shift)).
 8. The method of claim 1,further comprising: applying an orthogonal code to the sequence of thesecond reference signal.
 9. A method of receiving reference signals by afirst user equipment (UE) in a wireless communication system, the methodcomprising: receiving, from a base station, a sequence of a firstreference signal using a physical cell identifier of the base station;receiving, from the base station, a message including mode informationindicating a first mode for a coordinated multi-point (CoMP) operationand a predetermined identifier being different from the physical cellidentifier of the base station; and receiving, from the base station, asequence of a second reference signal for the first UE using thepredetermined identifier instead of the physical cell identifier of thebase station.
 10. The method of claim 9, wherein the message is receivedvia radio resource reconfiguration (RRC) signaling.
 11. The method ofclaim 9, wherein the first reference signal is a cell-specific referencesignal and the second reference signal is a UE-specific referencesignal.
 12. The method of claim 9, further comprising: receiving adownlink control information (DCI) including a field indicating an indexof the predetermined identifier.
 13. The method of claim 9, furthercomprising: obtaining the physical cell identifier via aprimary-synchronization signal (PSS) and a secondary-synchronizationsignal (SSS).
 14. The method of claim 9, wherein the sequence of thesecond reference signal is mapped on resource elements without physicalcell identifier based cell-specific frequency shifting (V_(shift)). 15.The method of claim 9, wherein an orthogonal code is applied to thesequence of the second reference signal.
 16. A base station comprising:a processor configured to generate a sequence of a first referencesignal using a physical cell identifier of the base station; and atransmitter configured to transmit, to a first user equipment (UE), amessage including mode information indicating a first mode for acoordinated multi-point (CoMP) operation and a predetermined identifierbeing different from the physical cell identifier of the base station,wherein the processor generates a sequence of a second reference signalfor the first UE using the predetermined identifier instead of thephysical cell identifier of the base station.
 17. A user equipment (UE)comprising: a receiver; a processor controlling the receiver to receive,from a base station, a sequence of a first reference signal using aphysical cell identifier of the base station, to receive, from the basestation, a message including mode information indicating a first modefor a coordinated multi-point (CoMP) operation and a predeterminedidentifier being different from the physical cell identifier of the basestation and to receive, from the base station, a sequence of a secondreference signal for the UE using the predetermined identifier insteadof the physical cell identifier of the base station.